Light-emitting diodes (LEDs) are semiconductor devices that convert electrical energy into electromagnetic radiation, including visible light. Due to their reliability, high luminous efficacy and low maintenance requirements, LEDs are increasingly being used in various lighting applications such as ambient lighting, signage, advertising, display lighting, and backlit lighting applications.
It is well known that light of a desired spectral composition or, in photometric terms, a desired chromaticity and luminous flux, can be generated by intermixing adequate amounts of light from different colour light sources. When light from, for example, different colour LEDs is intermixed, the chromaticity of the mixed light can be sufficiently accurately determined by characteristics such as the intensities, center wavelengths and spectral bandwidths of the LEDs.
The characteristics of LEDs can vary for a number of reasons, for example, device aging and/or fluctuations in device operating temperature. These variations can cause undesirable effects under operating conditions of the LEDs. Possible solutions include optical feedback control to monitor the luminous flux output of the different colour LEDs and to adjust the drive currents of the LEDs such that the luminous flux output and chromaticity of the light emitted by each LED or at least the mixed light generated by a group of LEDs remains substantially constant. Monitoring the emitted light requires some means of measuring the luminous flux output per LED colour or per LED, for example.
To date, several optical feedback solutions have been proposed to detect and evaluate the luminous flux output and chromaticity of the output light of a lighting device in order to monitor these characteristics. For instance, U.S. Pat. No. 6,600,562 teaches an array of photosensors each having a selected colour filter responsive to light of a selected colour. These photosensors however, are prone to optical crosstalk due to the overlap in the spectral radiant power distribution of the light emitted by various colours of LEDs. This optical crosstalk can reduce the accuracy of the light information collected by the photosensors.
U.S. Pat. No. 6,741,351 describes a LED luminaire with multi-channel colour sensors for optical feedback, wherein each channel is comprised of a broadband photosensor and a colour filter with transmittances that approximate that of the red, green and blue LED spectral radiant power distributions. Since the spectral radiant power distributions of the LEDs tend to overlap for the different colours, channel crosstalk is inevitable and can limit the performance of the optical feedback system.
A partial solution to this optical crosstalk problem is to select bandpass filters with narrow bandwidths and steep cutoff characteristics. Although satisfactory performance levels for such filters can be achieved using multilayer interference filters, these interference filters can be expensive and typically require further optics for collimating the emitted light, as the interference filter characteristics depend on the incidence angle at which the light impinges on these filters.
Another problem associated with interference filters is that the center wavelength of an LED depends on the LED junction temperature and this center wavelength can vary significantly depending on the type of LED. In addition, the bandpass transmittance spectra of interference filters are also temperature dependent. The output signal of the photosensor therefore depends on the spectral radiant power distribution of the LED as modified by the bandpass characteristics of the interference filter associated therewith. Hence there exist situations where the output signal of the photosensor may change with ambient temperature even if the LED spectral radiant power distribution remains constant, which can further limit the performance of the sensor system.
U.S. Pat. No. 6,127,783 describes how radiation from each LED colour is controlled by an electronic control circuit, which can selectively turn off the LEDs, which are the colours not being measured, in a sequence of time pulses and uses a single broadband optical sensor for detection. A problem with this approach is that colour balance is periodically and potentially drastically altered each time the LEDs are de-energized, thereby possibly causing noticeable flicker. Since the optical sensor requires a minimum amount of time to sense the radiant flux of the energized LEDs accurately and with an acceptable signal-to-noise ratio, the choice of sampling frequencies can be limited by the sensitivity and noise characteristics of the optical sensor. A limited sampling frequency can result in lower sampling resolution and longer response times for the optical feedback loop. Since light from no more than one LED colour is measured at a time, this approach for optical data collection can increase the feedback loop response time by about the number of different LED colours used in the system. For example, for a system with red, green, and blue LED clusters the response time can be multiplied by factor of about three, and for a system with red, green, blue, and amber LED clusters the response time can be multiplied by a factor of about four.
U.S. Pat. No. 6,445,139 describes a luminaire having a plurality of LEDs producing light of different colours. The light output of each colour is measured by an electronic control circuit that turns OFF the LEDs for the colours not being measured in a sequence of time pulses. The average light output during the measuring period is substantially equal to the nominal continuous light output during the ordinary operation to avoid visible flicker. Similarly, U.S. Pat. No. 6,495,964 seeks to alleviate the flicker by selectively measuring the light output of the LEDs in a sequence of time pulses, whereby the current for the colour being measured is turned off. Neither of these proposed solutions, however, addresses periodic and potentially drastic changes in colour balance or degradation in feedback loop response time due to the deactivation sequences required for light sampling.
As described in U.S. Pat. No. 6,596,977, the light output of the LEDs is sampled by a broadband optical sensor during PWM drive current pulses whenever the drive current has reached full magnitude. This procedure can avoid the effect of the rise and fall times of the PWM pulse. The average drive current can then be determined by low pass filtering. A difficulty associated with this approach can be that the PWM pulses must be synchronized such that at least one LED colour is de-energized for a finite period of time during the PWM period. This requirement can prevent operation of all different colour LEDs at full power at 100% duty factor. Another disadvantage associated with the average light sensing method is that the sampling period typically must provide sufficient time for the optical sensor to reliably measure the radiant flux of the energized LEDs. In addition this light sensing method requires that the LED colours are to be measured sequentially, which can limit the feedback loop response time.
Based on the above, there is a need for a new method and apparatus for light intensity control for a luminaire.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.